Anti-Reflective Coatings (ARC)
The presence of a thin coating layer on a substrate provides two interfaces, the first is between the air and the thin layer and the second is between the thin layer and the substrate. Each of these interfaces reflects the light beam and generates a reflective beam. If the coating is a quarter wavelength thick and has an index of refraction less than that of the substrate then the two reflections are 180° out of phase and may therefore cancel each other by means of destructive interference.
The refractive index of a thin layer coating on a substrate should be substantially lower than that of the substrate (on to which it is applied) and its thickness should roughly be ¼ of the desired non-reflecting wavelength so as to obtain an anti-reflective coating. In the more complex cases, ARCs consist of a film comprising multiple thin alternating layers of contrasting refractive indexes.
Anti-reflection coatings are widely used in various applications such as display panels, solar cells and optical devices. Depending on the application, various types of ARCs may be used. Generally, there are at least two categories of ARCs available including, inter alia:
(i) A quarter-wavelength ARC that eliminates reflection of an essentially monochromatic wavelength.
As an example, Tustison et al. describe in U.S. Pat. No. 4,907,846 an ARC for an IR transparent optical element wherein the thickness of the anti-reflective layer is a quarter of the wavelength which must be maximally transmitted through the optical element.
(ii) Multiple coating (multicoating) configurations that accommodate a wide wavelength region of the spectrum.
As an example, Biteau; John et al. and Arrouy Frederic et al describe in, respectively, US patent applications publications Nos. 2006/275627 and US 2008/0002260 the use of anti reflecting multicoating.
Anti reflectance may be interfered by accumulation of dirt on its surface. The dirt on the surface of the ARC absorbs part of the light that otherwise would be transmitted trough the ARC and the substrate. This problem is especially intensified when the ARC has a polar surface which attracts dirt by electrostatic forces. Thus, frequent cleaning of the ARC is required to preserve the antireflective advantage it provides to the application.
Self Cleaning Surfaces or Coating (SCC)
Solid surfaces can be classified depending on their contact angle. A surface having a contact angle lager than 90° is considered hydrophobic, larger than 150° is considered ultra-hydrophobic; smaller than 90° is considered hydrophilic; and close to zero is considered ultra-hydrophilic
Self cleaning surfaces exist in nature. Even when rising from muddy waters, the Lotus flower stays clean and untouched by pollution. To achieve this natural self-cleaning effect, the Lotus flower presents two basic features: (a) an ultra-hydrophobic surface chemistry, i.e. the Lotus has a contact angle greater than 150° such that water drops roll off and do not evaporate in the surface, thereby leaving no stains; and (b) a special nanometric morphology that acts like a “Fakir bed” reducing the contact area of the water drop to the surface, thus reducing the adhesion forces between them and allowing the water drop to slide down the Lotus leaf surface and clean the dirt upon it.
Hydrophobic coatings are mainly silicon or fluorinated coatings. Silicone coatings are mostly based on organosiloxane polymers (polysiloxanes). These are hybrid material composed of pendant organic groups attached to an inorganic siloxane backbone like polydimethylsiloxane (PDMS). Polysiloxanes typically achieve low free surface energy around 22-23 dyne/cm and contact angles ranging from 90° to 100°.
Fluorinated coatings have a very low surface energy (18 dyne/cm), are resistant to chemicals and present good thermal stability. Polytetrafluoroethylene (Teflon) coatings show contact angles close to 110°.
A few patents deal with the subject of hydrophobic surfaces. Smith, et al.; describe in U.S. Pat. Nos. 5,736,249 6,084,020; 6,120,849; and 6,153,304 non-sticky, non-fouling and ice-phobic hydrophobic coating systems for applications on inorganic, organic and metallic substrates, by means of two coating layers. The first layer is a fluoro-copolymer having good adhesion properties to the substrate and the second layer a siliconic polymer having hydrophobic properties, e.g. a surface energy in the range of 18-21 dynes/cm and contact angles in the range of 90°.
Arpac, et al. describe in U.S. Pat. No. 6,291,070, a method for producing nanostructured molded articles and layers by means of a wet chemical process comprising a free flowing composition containing solid nano-scaled inorganic particles having polymerizable and/or polycondensable organic hydrophobic surface groups. The molded articles are easy to clean.
Hashimoto, et al. describe in U.S. Pat. No. 6,524,664 a method for photo-excitation of a coating layer of a photocatalytic metal oxide in a manner that yields a mosaic of hydrophilic and hydrophobic regions in the exposed area. According to Hashimoto, et at the resulting surface is amphiphilic such that oil deposited on it can be easily removed by rinsing with water and water (or water-based liquids) deposited on it can be easily removed by rinsing with oil.
Yamazaki, et al. describe in U.S. Pat. Nos. 6,420,020, and 6,531,215 an antifogging article made of an antifogging film on a substrate. The antifogging film contains a first layer of a water absorbing organic polymer (acetalized polyvinyl alcohol) and a second layer of an inorganic water repellent silicon compound.
Nun, et al. describe in U.S. Pat. No. 6,811,856 a self-cleaning surface which has, an at least partially hydrophobic nanotextured surface structure made from elevations and depressions, where the elevations and depressions are formed by particles secured to the surface, wherein the elevations and/or depressions in the nanometer range (with average height from 20 to 500 nm and distance bellow 500 nm). The hydrophobic properties can be obtained by pretreatment of the particles with hydrophobic compounds (e.g. alkylsilanes and fluoroalkanes) or after the particles have been secured on to the substrate surface by dipping or spraying the surface with hydrophobic substances.
Fan et al. describe in U.S. Pat. No. 6,913,832 the production of nanostructured multilayered surfaces using organosilanes and silsesquioxanes.
Very few attempts have been made to combine antireflective with self cleaning properties. Rubner et. al. describe in US patent application publication Nos. 2008/268229 and 2007/104922 the combination of super hydrophilic coating with antireflective and antifogging properties.
Akira and Shinton in JP 2006/162711 disclose a composite material comprised of nanometer-sized titanium oxide particles that are bound to sub micron silicon dioxide particles having a combination of anti-reflective and self cleaning properties.
Finally, Brian G. Prevo, Emily W. Hon and Orlin D. Velev describe in J. Mater. Chem., 2007, 17, 791-799 the assembly of colloidal silica nanoparticles into antireflective coatings (ARCs) and onto polycrystalline silicon solar cells. The nanocoatings reduced the reflectance of the solar cells by approximately 10% across the near UV to near IR spectral range, which provided a 17% increase in the output power of the devices. The publication suggests modification of the silica based coatings by attachment of monolayers of fluorosilanes, which may make them superhydrophobic and/or self-cleaning